A headrest device is mounted on a vehicle, which a headrest device mitigates an impact applied to a cervix by restricting a backward movement of a head of an occupant after moving forward when the vehicle is rear-ended or when that the vehicle is to be rear-ended is predicted.
For example, a headrest device described in JPH11-334439A (FIG. 2) includes a drive motor provided in a headrest, a worm fixed to a rotational shaft of the drive motor, and a gear that is engaged with the worm. The drive motor is activated by signals transmitted from a sensor which is mounted on a vehicle to predict or detect rear-end collision. With this headrest device, when the collision is either predicted or detected, the headrest is tiled forward by activating the drive motor to protect a head of an occupant and to prevent the occupant from causing hyperextension-hyperflexion injury when the vehicle is rear-ended. Further, with a headrest device described in JPH08-187139A, a stepping motor and a gear mechanism vibrates stays which support a headrest and an attitude of the headrest is fixed when the vehicle is rear-ended, in order to prevent the occupant form causing hyperextension-hyperflexion injury.
Considerable load is applied to the rotational shaft of the motor and the gear when a head of an occupant impacts on the headrest upon the rear-end collision. This considerable load can rotate the motor in reverse, which leads to a risk that the load applied at the rear-end collision cannot be supported, that is, the head of the occupant cannot be supported. In addition, using a larger size motor which outputs a torque having a degree which can resist the load, that is, the motor does not rotate in reverse by the load applied at the rear-end collision, hinders downsizing and lightening the weight of the headrest device.
A need thus exists for a headrest device, which securely supports a head of an occupant, prevents a load by an impact of the head against a headrest from being applied to a drive portion of the headrest, and downsizes and reduces the weight thereof.